FIELD OF THE INVENTION
This invention relates generally to carbon substrates and methods for producing preforms using the substrates, and in particular, to a needled polar woven fabric substrate and method for producing a carbon preform using the substrate for friction disk applications.
A brake disc for an aircraft or an automobile requires a material having high thermo-mechanical resistance and long wear. For some applications, asbestos is used due to its heat resistance properties. In addition to asbestos, carbon may also be used because of its good heat resistant properties, although conventional carbon brake products are expensive and historically have been restricted to aerospace or automotive racing applications.
Generally, a substrate of carbon fiber or a carbon precursor may be used to produce a conventional carbon/carbon part with sufficiently high heat resistance values for use in, for example, an aircraft braking system. These conventional parts require a complicated time consuming process to produce a part with sufficient carbon to provide the necessary high temperature characteristics. These conventional carbon/carbon parts are expensive due to the complicated manufacturing process. There are a number of different types of substrates used to make conventional carbon parts including discontinuous carbon fiber molding compound, non-woven air lay carbon fiber substrates, woven carbon fiber substrates, or braided carbon fiber substrates.
To produce a conventional carbon part from a carbon fiber substrate, a plurality of carbon fiber precursors, such as polyacrylonitrile (PAN) fibrous layers, are used. These substrates are sheets of material that must be cut into a particular shape, such as a disk for a brake, which wastes a portion of the substrate because the substrate is generally not recyclable. These substrates may be stacked on top of each other to a desired thickness and then the stacked substrates may be needle-punched together to join or consolidate the substrates to each other by intermingling carbon fibers between the layers of the substrates. This consolidation of the substrates creates a preform. The preform may then be batch carbonized, in which the preform is placed in an oven at 800 to 1100 degrees Celsius, to char the fibers of the substrate and increase the carbon content of the preform. These preforms may then have additional carbon atoms deposited on the carbon fibers of the preforms by using a chemical vapor infiltration (CVI) process. In the CVI process, the preform is placed in chamber, the air in the chamber is removed, and a carbon bearing gas, such as methane, is introduced into the chamber which when subjected to temperature releases carbon atoms that settle/infiltrate into the preform. The CVI process may increase the carbon content and density of the preform. The preform may then be heat treated to reorient the carbon atoms to a more energetically favorable configuration to improve, for example, physical properties, machined, and treated with an anti-oxidant to form the finished carbon part.
The conventional preform process, as described above, and the conventional carbon parts have several problems. The batch carbonization process is slow and time consuming, taking hours or days which increases the cost of the part. There is also the added cost of the furnaces necessary to perform the carbonization step because these furnaces cannot be used for any other purpose. In addition, any material removed from the preform during the die cutting and shaping processes cannot generally be reused because there is no appropriate method for recycling this scrap material back into the preform manufacturing process. Thus, due to the above problems, the conventional preforms produce a carbon part that is typically too expensive to use for most commercial applications.
Braiding is another conventional method which attempted to reduce the waste material associated with each preform. A tubular braid of textile may be collapsed to form a flat tape and the flat tape may be helically wound to form a disk preform with reduced waste since the disk is not cut out from a sheet substrate. The distribution of the fibers is not homogeneous in the preform using this method.
Another conventional substrate uses carbon fibers that are impregnated with a suitable binder and then the impregnated substrate may be compressed under heat and pressure to form the near net shape preform. The preform is then batch carbonized to char the binder via condensation of the binder into carbon. The binder may be liquid furfuryl alcohol polymer catalyzed with maleic anhydride. Once again, this substrate requires a batch carbonization process step in order to char the binder. Still another substrate for a carbon part uses carbon fibers, that may be oxidized polyacrylonitrile (PAN) fibers that may then be carbonized to form the carbon preform that may be subjected to the chemical vapor infiltration (CVI) process. This substrate also requires a carbonization step.
These conventional materials for producing carbon parts require a batch carbonization step, which is slow, increases the cost of the final part and requires an expensive carbonization furnace. None of the conventional materials provide a uniform, homogeneous, controllable fiber orientation distribution over an entire preform. The conventional preforms also waste substrate material which increases the cost of the preform. Due to these deficiencies, conventional preforms produce carbon parts that are too expensive to be used in most conventional commercial applications and do not provide sufficiently uniform fiber distribution.
Thus, there is a need for a preform manufactured from a polar woven substrate and a method for producing carbon preforms using a substrate which avoids these and other problems of the known substrates, processes and carbon reforms, and it is to this end that the present invention is directed.